MIMO (Multi Input Multi Output) is a communication system using a plurality of transmit antennas and a plurality of receiving antennas. The MIMO system may increase channel capacity linearly with the number of transmit and receiving antennas, without an additional increase in frequency bandwidth. There are two types of MIMO schemes, transmit diversity and spatial multiplexing. Transmit diversity increases transmission reliability by transmitting symbols in a plurality of channel paths, while spatial multiplexing increases transmission rate by transmitting different data streams simultaneously through a plurality of transmit antennas.
MIMO schemes may also be classified into open-loop MIMO and closed-loop MIMO depending on whether a transmitter has knowledge of channel information. Open-loop MIMO does not require that the transmitter is aware of channel information. In contrast, the transmitter has channel information in closed-loop MIMO. The performance of a closed-loop MIMO system depends on how accurate channel information the transmitter gets.
Channel information is information about radio channels between a plurality of transmit antennas and a plurality of receiving antennas (e.g. attenuation, a phase shift, a time delay, etc.). Many stream paths exists according to the combinations of the transmit and receiving antennas and channel status fluctuates over time in the time and frequency domains in view of a multipath time delay, which is called fading, in the MIMO system. Accordingly, a receiver calculates the channel information through channel estimation. Channel estimation is the process of estimating channel information required for recovering a distorted transmission signal. For example, the channel estimation is equivalent to estimation of the amplitude and reference phase of a carrier. In other words, the channel estimation is to estimate the frequency response of a radio link or a radio channel.
For channel estimation, a reference value may be estimated from several Reference Signals (RSs) received from the transmitter using a channel estimator. An RS is symbols transmitted at a high power level without carrying actual data to help channel estimation at the receiver. Both the transmitter and the receiver may perform channel estimation using RSs. Specifically, the RS-based channel estimation is to estimate a channel using symbols known to both the transmitter and the receiver and recover data based on the channel estimate. An RS is called a pilot signal.
In the meantime, 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) systems are standardized in such a manner that a single antenna is used for uplink transmission from a User Equipment (UE) to a BS. A demodulation RS (DMRS) based on Cyclic Shift (CS) is defined in uplink single antenna transmission. However, 3GPP LTE-Advanced (LTE-A) systems are required to support multi-antenna transmission even for uplink transmission.
To support uplink MIMO transmission, an RS design scheme is needed which improves channel estimation performance for MIMO transmission while maintaining subframe design used for single antenna transmission supported by the LTE system and backward compatibility.